The field of the invention is synthetic resin polymerization and the present invention is particularly concerned with preparing polylauryllactam low in fisheye gel content by polymerizing lauryllactam at elevated temperatures in the presence of water and possibly of monoor dicarboxylic acids as chain regulators, the procedure taking place in a single step polymerization at temperatures ranging from 260.degree. to 340.degree.C or in a two-step polymerization at temperatures ranging from about 260.degree.to 340.degree.C during the first step thereof and from about 200.degree. to 300.degree.C in the second step of post-condensation.
The state of the prior art of polymerizing lauryllactam (nylon-12) may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", Vol. 16 (1968), pages 88-105, particularly p. 92 and U.S. Pat. No. 3,799,899, the disclosures of which are incorporated herein. The 2-hydroxphenylbenzoxazole used as an additive in the present invention is disclosed in French Pat. No. 1,241,329 and this disclosure is also incorporated herein.
It is known to the prior art to prepare polylauryllactam in the presence of water and possibly in the presence of chain regulators, the procedure taking place in a first step at a temperature between about 200.degree. and 340.degree.C and in a second step between about 270.degree. and 340.degree.C as disclosed in German Published Application No. 1,495,149 and French Patent No. 1,413,397. The products so obtained have a large fisheye gel content. These fisheye gels interfere in the manufacture of yarns, monofilaments and especially films plagued with opaque thickenings, whereby the films have an unpleasant appearance and act inhomogeneously, while the ease of printing the film becomes hampered or altogether impossible.
It is also known to the prior art to carry out a single step polymerization of lauryllactam in the presence of catalysts of the strong mineral acid type such as phosphorous acid, phosphoric acid or sulfonic acids, at temperatures ranging from about 280.degree. to 300.degree.C as disclosed in German Published Applications Nos. 1,520,551; 1,907,032; and 1,495,147, or, in a first step at temperatures exceeding 300.degree.C and then possibly post-condensing in a second step at temperatures below the melting point of the polylauryllactam, as disclosed in German Published Application 1,267,428. However, operating with such strongly acid catalysts entails the drawback of the polylauryllactam so prepared suffering from reinforced hydrolytic degradation because the acid catalysts used remain in the polymer and furthermore, the fisheye gel content in this case is also high.
It is also known to the prior art to prepolymerize lauryllactam in a first step at a temperature between about 265.degree. and 320.degree.C and to postpolymerize in a second step at a temperature lower than that of the first step, i.e., between about 220.degree. and 265.degree.C as disclosed in U.S. Pat. No. 3,799,899. Polylauryllactam with only a slight fisheye gel content is obtained by this process. However, longer reaction times are generally required than for the operation at higher temperatures, so that there is a drop in the space time yield. Again, in the case of operational failures, the polylauryllactam is exposed to higher temperatures for fairly long times, with the resulting occurrence of fisheye gel specks.
Numerous compounds are known for stabilizing polyamides, these compounds essentially being used as light and oxidation stabilizers. No reduced fisheye gel formation has been observed. German Published Application No. 1,694,473 merely points out that copper compounds in combination with lithium iodide for the purpose of polyamide heat stabilization supposedly shows simultaneous evidence of a lessened tendency to fisheye gel formation. However, these polyamides are strongly colored and the examples listed refer solely to polyamide-6,6 and this finding could not be substantiated for polylauryllactam.
According to U.S. Pat. No. 3,799,899, lauryllactam is heated in a mixture with about 0.5 - 50 percent by weight, preferably 1-10 percent by weight, of water, based on the amount by weight of lauryllactam, in conventional pressure vessels for about 2-20 hours, preferably 5-10 hours, to about 265.degree.-320.degree.C, preferably 270.degree.-290.degree.C. During this process, an internal pressure of up to about 70 atmospheres gauge is attained in the reactor in dependence on the temperature, the amount of water added, and the free gas volume. Advantageously, the procedure is carried out at internal pressures of about 10-30 atmospheres gauge. Suitably, an inert atmosphere as provided by a protective gas, such as nitrogen, is used in the process.
After the preliminary polymerization of U.S. Pat. No. 3,799,899, the water is removed by expansion. Thereafter, a post polymerization is conducted at about 220.degree.-265.degree.C, preferably at 240.degree.-265.degree.C under a stream of a protective gas, for example nitrogen, until the desired viscosity has been attained, for example for a period of about 0.5 to 10 hours under atmospheric pressure.
In order to regulate the molecular weight of U.S. Pat. No. 3,799,899, it is possible to add to the lauryllactam, prior to or during the polymerization, the usual chain regulators, such as monocarboxylic acids having 2 to 18 carbon atoms or polycarboxylic acids having 2 to 12 carbon atoms, particularly dicarboxylic acids having 2 to 12 carbon atoms, in amounts of about 0.05-2 molar percent, preferably 0.1 - 0.5 molar percent. Examples of carboxylic acids which can be used are acetic acid, propionic acid, stearic acid, adipic acid, sebacic acid, azelaic acid, or decanedicarboxylic acid, especially adipic acid and sebacic acid.